Current spreading in AlGaN:Mg cladding layers of laser structures
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Current spreading in AlGaN:Mg cladding layers of laser structures Stephan Figge1, Tim Böttcher1, Christoph Zellweger2, Marc Ilegems2, Detlef Hommel1 1 University of Bremen, Institute of Solid State Physics Kufsteiner Str., 28359 Bremen, Germany 2 Ecole Polytechnique Fédérale de Lausanne, Institut de micro- et optoélectronique EPFL, Lausanne Switzerland ABSTRACT The consequences of the anisotropic resistance in AlGaN/GaN strained layer superlattices for the operation of laser diodes were studied for structures driven in LED mode. A series of laser structures containing different Mg-doped AlGaN bulk and AlGaN/GaN strained layer superlattice cladding layers was compared to estimate the current spreading in the cladding due to the formation of 2D-hole gases. Current-voltage measurements revealed a significant spread of the current path in the superlattices, whereas no spreading was seen for the bulk cladding. In contrast to this, investigations of the electroluminescence showed no significant differences. INTRODUCTION AlGaN is commonly used for cladding layers in GaN-based laser structures, but due to the tensile biaxial strain state of AlGaN grown on GaN, these layers tend to release the strain energy via the formation of cracks [1]. AlGaN/GaN strained-layer superlattices (SLS) are seen as a potential way to extend the range of possible Al concentrations [2]. For the case of highly pdoped SLS the formation of a 2D-hole gas [3] is a major drawback as it leads to an anisotropy of conductance and therefore to current spreading in laser structures, thus reducing the effective current density in the active region. EXPERIMENTIAL We grew different p-type cladding layer structures on top a template that contains the lower part of a laser structure including the upper p-type waveguide. The samples were deposited by metalorganic vapour phase epitaxy (MOVPE) in a vertical close-spaced showerhead reactor. The template is based on a conventional GaN-on-sapphire structure starting with a low temperature nucleation layer and a 2.5 µm thick GaN buffer layer. The upper 1 µm of the buffer has been Si-doped with a free electron concentration of 1x1018 cm-3. The n-type cladding layer consists of an AlGaN/GaN SLS with 100 periods and an overall thickness of 500 nm. The average aluminum content is 7%. The active region contains a 3-fold InGaN/GaN:Si multi quantum well (MQW) on top of a 100 nm thick Si-doped waveguide layer. No electron blocking layer was grown atop the MQW in order to minimize current spreading at the interfaces. The template was capped by a 100 nm thick Mg-doped GaN waveguide layer. To remove oxides from the surface the templates have been thermally etched in-situ before regrowth and then three different cladding structures have been applied as follows: (1) A bulk cladding, (2) a SLS which was continuously doped and (3) a semi-δ-doped SLS. The latter was only doped in the GaN wells of the SLS with an additional δ-doping at the start of the well to maintain the same total amount of Mg in the structure. All claddings have an
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